1. Field
This invention is in the field of proximity focused, night vision image intensifiers. Specifically, this invention relates to image intensifiers that produce electrical output signals.
2. Related Art
Intensifiers include, but are not limited to, electron bombarded active pixel sensors (EBAPS) (U.S. Pat. No. 6,285,018 B1) and electron bombarded charge coupled devices (EBCCDs). U.S. Pat. No. 6,285,018 is incorporated by reference into the disclosed background for this patent. These sensors fall into a class of vacuum imaging sensors that predominantly use proximity focused electron optics. Proximity focused sensors typically use planar photocathodes and planar anodes. The image information contained in the intensity pattern of the electrons emitted from the photocathode is transferred across the vacuum gap of the sensor by accelerating the electrons through an electric field. The electric field is established by biasing the photocathode and the anode to different voltages. Typical bias voltages for EBAPS internal components are −1200V on the photocathode and 0V on the anode assembly. As photoelectrons traverse the vacuum gap, they spread from their emission position on the photocathode to a proximate but not exactly translated impact position on the anode assembly. This spreading results in a loss of image sharpness. This loss of image quality is minimized by minimizing the transit time of the electrons across the vacuum gap. Transit time is in turn minimized by minimizing the cathode to anode gap. The improvement in transit time at a given bias voltage must be weighed against other performance attributes that tend to degrade with increasing electric field strength. Specifically, photocathode dark current emission tends to increase with increasing electric field strength. Increased photocathode dark current adversely affects image intensifier performance when used for night vision applications. Typical electric fields employed over photocathodes for proximity focused night vision image intensifiers range from ˜3000 to ˜8000V/mm. Accurate control of the electric field strength translates into precise dimensional requirements for the components used to manufacture image intensifiers. Specifying precise dimensional tolerances for image intensifier components generally raises production costs for these components.
Anode assemblies for indirect view image intensifiers including EBAPS, EBCMOS and EBCCDs may incorporate collimating structures. U.S. Pat. No. 8,698,925 B2 is incorporated by reference to this patent to document and set a basis for this aspect of the prior art.
One approach image intensifier manufacturers have attempted to use in the past is the use of a spacer attached to the photocathode to specify the vacuum gap that lies immediately above the photocathode and across which the electric field is applied. Iosue (U.S. Pat. No. 6,847,027 B2) describes the use of an insulating spacer which is fabricated as an integral portion of the photocathode manufacturing process. Although the described manufacturing process and structure may achieve the goal of setting a minimum limit to the vacuum gap overlying the photocathode, the design suffers from a number of shortcomings. Perhaps the most important of these issues is cost. The generation of glass bonded photocathodes is as described by Iosue, a relatively complex process. The incorporation of a spacer as an integral piece of the photocathode increases the required handling and processing of the photocathode assembly. The GaAs photoemission surface is quite sensitive to damage and contamination. Increasing the complexity of the manufacture process and the required handling translates into increased component yield loss and consequently increased cost. Additionally, Iosue fails to address issues related to the physical compliance of the surface that is contacted by the spacer. Kennedy (U.S. Pat. No. 4,178,528) describes a room temperature Indium seal as is typically used on image intensifiers as employing forces on the order of 150-200 pounds of force per square inch. During the application of this force the Indium used to insure the vacuum seal between the window and vacuum body assemble is displaced as the gap between the photocathode and an opposing surface is reduced. The perspective to be gained from the previous description is that the force required to damage an MCP as used in the image intensifier described by Iosue or the anode assembly of the present invention is much lower than the force applied to affect the vacuum seal. Consequently, the force versus compliance characteristics of the surface opposing the photocathode during seal specifies the accuracy with which the opposing component must be placed with respect to the photocathode stopping point in order to avoid damage. A failure to design in sufficient compliance will potentially result in: low sensor yield (Adds cost), tight geometric specification requirement for sensor components (Adds cost), and inconsistent forces between the photocathode and the opposing surface present the potential for shock/vibration damage and reliability issues particularly when high voltage gated gain control approaches are used.
Indirect view image intensifiers such as MCP-CMOS (as described in U.S. Pat. No. 7,880,128), EBCCDs (U.S. Pat. No. 6,281,572 Robbins) or EBAPS (U.S. Pat. No. 7,607,560) typically employ multi-layer ceramic headers which constitute a portion of the vacuum package to support the semiconductor anode assemblies. A large variety of approaches have been employed to mount semiconductor die within proximity focused image intensifiers as illustrated by the cited patents. However, with the exception of U.S. Pat. No. 7,607,560, none of the prior art indirect view image intensifier packaging approaches include compliant anode assemblies which index directly to the photocathode assembly. In the case of U.S. Pat. No. 7,607,560, the compliant anode assembly is accomplished via the use of molten braze or solder material between the anode assembly and the vacuum package at the time the photocathode is sealed against the vacuum package assembly. This requirement adds image intensifier processing constraints that are undesirable. Specifically, accurate vacuum temperature control is difficult to accomplish in the hardware required to generate the vacuum seal. Additionally, any jostling during the vacuum sealing process can result in an uncontrolled displacement of the molten braze/solder material resulting in a non-functional image intensifier.